Hydroxyindole-O-methyl transferase activity of male rat pineal glands following hypophysectomy and HGG treatment

The effect of thyroid hormones on rat pineal indoleamine metabolism in vitro

The effect of TSH, triiodothyronine (T3) and thyroxine (T4) on basic and noradrenaline (NA)-induced indoleamine metabolism was investigated in rat pineal culture based on the observation that, under the experimental system used, melatonin is formed from its amino acid precursor tryptophan. When added to the culture medium, alone or in the presence of NA, TSH in dosages of 5 and 20 mU had no significant effect on pineal tryptophan metabolism.T4 and 2.5 microgram increased the melatonin concentration and, in the presence of NA 10(-4) M, also induced an increase in N-acetylserotonin (NAS). T3, already at a dosage as low as 0.25 microgram enhanced the melatonin concentration, while 2.5 microgram produced significant increases in concentrations of all pineal indoleamines measured. The latter dosage of T3 also enhanced the NA-stimulated NAS. On adding larger quantities of T4 or T3 to the medium none of the changes encountered with the lower dosages could be seen. The results obtained may suggest a direct positive feedback between the thyroid and pineal glands.

Pineal and serum melatonin at midday and midnight following pinealectomy or castration in male rats

Melatonin has been extractkd by chloroform from rat serum and quantified by the Rana pipiens tadpole bioassay. Like pineal melatonin, serum melatonin was high at mid-dark and low at mid-light. This finding suggests a diurnal rhythm of serum melatonin in the rat. Serum samples obtained at mid-dark had melatonin levels ranging from 0.02 to 0.05 ng per ml of serum. When rats were pinealectomized, melatonin activity in the serum was abolished. Thus, rat serum melatonin may originate primarily from the pineal. There was no significant change in pineal melatonin content nor in serum melatonin concentrations 7 or 12 days following orchidectomy.